Alternating current light-producing system and method of operating same



Man-ch26, 1940. E. R. GEIB ET AL 2,195,329

ALTERNATING CURRENT LIGHT-PRODUCING SYSTEM AND METHOD OPERATING SAME Filed Aug. 25, .1938

Patented Mar. 26, 1940 UNITED STATES PATENT OFFICE ALTERNATING CURRENT LIGHT-PRODUC- ING SYSTEM AND METHOD OF OPERAT- ING SAME Application August 25,

15 Claims.

This invention relates to an alternating current light-producing system and particularly to a method of operating such a system, in a pre-' ferred embodiment to produce a low-voltage highintensity are between a pair of electrodes, which system is characterized in that it provides an are which is very steady and contains a source of light of high intrinsic brilliancy concentrated into a relatively small volume near the electrode tips. Preferably, cored carbon electrodes are employed and the core materials are so selected that the light energy produced in accordance with this invention will furnish, in preponderance, radiation within selected portions of the visible or invisible spectrum, for such uses as motion picture projection, therapeutic or biological treatments, and industrial applications such as the irradiation of foods and other materials. The principles of the invention are specifically described herein as applied in conjunction with optical systems for projecting motion pictures. This application contains subject matter in common with our copending application Serial No. 667,420, filed April 22, 1933.

In the projection of motion pictures by means of small volume since diffused light or light of low intrinsic brilliancy cannot be sharply focused in the optical systems of projection devices. Ad-

ditionally, the light source must be stable as to position and it must furnish light of constant intensity. This second requirement is imposed by the necessity of illuminating uniformly the screen area and to prevent substantial variation in its brilliance. 'The proper color of the light is another requirement for the satisfactory projection of pictures. For ordinary black and white and for some types of color films, the light should be preponderantly blue-white in color; other types of color films require light containing a substantial amount of red or of those colors in or near the red portion of the spectrum.

It has not heretofore been possible to meet all of the foregoing requirements through the use of an alternating current arc light source, and an important object of this invention is the elimination of difliculties previously encountered and-the provision of a light-producing system including an alternating currentarc capable of fulfilling in high degree the conditions above set forth.

Other objects of this invention include the provision of such an alternating current arc-producing system requiring simple and inexpensive electrical equipment and which consumes relatively small quantities of electrical energy; the provision of such a light-producing system including a 1938, Serial No. 226,6"18

steady and uniform alternating current high-intensity are characterized in that it contains a source of light of high intrinsic brilliancy concentrated into a relatively small volume near each of the electrode tips; and the provision of such a light-producing system including an alternating current low-voltage high-intensity are having portions of radiant energy concentrated into a relatively small volume near the tip of each electrode, and in which the light source may be caused predominantly to supply radiation within selected portions of the visible or invisible spectrum. Q

The above and other objects and novl features of the invention will be apparent from the following description and the accompanying drawing, in which: I

Fig. 1 is a diagrammatic illustration of an alternating current light-producing system embodying the principles of the invention;

Figs. 2, 3 and 4 illustrate modifications of the embodiment shown in Fig.1; and

Fig. 5 diagrammatically illustrates the type of alternating current are which is produced between the electrodes of Figs. 1 to 4.

In accordance with the principles of this invention, three conditions must prevail in order to accomplish the desired results. N First, it has been found that the alternating current are must be operated under high intensity conditions. For this reason, carbon electrodes capable of operating at relatively high current densities are employed and for maximum constancy, the arc should be short, operated. at low voltage.

Second, the carbon electrodes preferably should be provided with a core containinga considerable amount of flame-producing material. Satisfactory results have been obtained with carbon electrodes, each of which is provided with a core containing flame-producing material of an amount more than about of the entire weight of the electrode. The maximum amount of flame-producing substances is governed solely by the limitations of manufacturing cored electrodes which are capable of being uniformly consumed in service. Practically, this amount is about 35% of the electrode weight. (These percentages refer to the carbon of the electrode plus its core material and do not include copper sheathing or other con- Third, it is essential to operate these cored electrodes, which contain a substantial amount of flame-producing material, at an alternating current density of more than about 550 amperes tively small volume near each of the adjoining.

electrode tips. While the above-described electrodes give satisfactory results at current densities as low as .550 amperes per square inch, better results are obtained for motion picture projecting systems with current densities of at least about 700 amperes per square inch up to about 1200 amperes per square inch. Of course, the abovenoted are voltage range will-vary slightly de;- pending upon the diameter of the electrodes and type of circuit employed, and this range applies to electrodes of about 7-8 mm. diameter in circuit with a high reactance transformer, the noload voltage of which is about 40% higher than the load voltage.

intended to indicate the approximate va1ues.of

This voltage range is only voltagewhich should be maintained in order to insure good operation under the specific circumstances stated.

A particularly desirable phenomenon occurswhile operating the above-described electrodes within the above-mentioned range of' current ,density of about 1200amperes per square inch,

the ends or faces of the electrodes assume a slight concave form. The current density at which this concavity begins to appear depends to some extent on the amount and nature of the flameproducing material employed and the size of the cores of the electrodes.

In the economical operation of the light-producing system of this invention using such high I light-producing system embodying this invention comprises a pair of cooperating cored carbon electrodes l0 and II between which an arc l2 can be established. The electrodes l0 and H fornr part of an electric circuit andare connected, respectively, to conductors l3 and I4 which, in turn, are connected to asecondary winding ii of a high-reactance step-down transformer l6. Suitable conductors l8 and I! leading from a source of suitable alternating current supply are connected to the primary winding ll of the transformer l6.'

Instead of employing a transformer having enough reactance to provide adequate arc stability, it may be desirable in some cases to employ a standard step-down transformer, and to provide additional impedance in series relation with either the primary or secondary winding of the transformer. In Figs. 2, 3 and 4 of the drawing are shown various ways in which the additional impedance may be provided in series with the transformer. In Fig. 2 additional impedance to improve arc stability is provided by connecting a resistor in series with the electrodes l0 and II.

. In Fig. 3 a similar result is obtained by inserting a reactor. 2| in-thecircuit in place of the resistor 1'20 of-FigQ2. Fig. 4' shows another modification of the arrangement in which both a resistor 22 and a reactor 23 are connected in series with the electrodes II and II.

By energizing the electrodes in the manner described, considerable economy is effected in the operation of the alternating current arcs at the high current densities required in the practice of this invention. Moreover, the power factor of the electrical energy consumed by such systems is relatively high.

Fig. 5 is anenlarged view somewhat diagram matically illustrating cooperating portions of the cored carbon electrodes II and II and a type of arc l2 which is produced in accordance with the principles of this invention. The electrodes III and II may be provided with c pper coatings I. and II. to facilitate the supply of alternating current,of high amperage to the electrodes at points near their tips. As the carbons are-.consumed the copper coating continually melts away, so that it is never sumciently close to the tip of the carbon to enter the'arc stream. The lowvoltage high-intensity arc I2 comprises portions 26 of light of, high intrinsic brilliancy concentrated into a relatively small volume near each of the electrode tips; a. ball-shaped portion 21 of lower intrinsic briiliancy; and tail flame portions 2!. The portions 26 constitute the'concentrated sources of usable light and they may be used jointly or separately dependingupon the nature of the projecting apparatus to be employed. In order to maintain the above-described characteristics of the are II, it is necessary to observe certain limitations relative to arc length ;and arc voltage. If the arc length becomes too short, the resiStanceof the arc decreases, consequently the voltage across the arc decreases and the current rises causing the cop r coatings l0 and H to melt away too rapidly and too far from the electrode tips, thereby rendering the arc unsteady and the same may blow out. On the other hand, if thearc length becomes too long, the resistance of the arc gap increases, less current flows, the light becomes less intense, and the arc flops" about the electrode tips.

- To further illustrate the invention, the following table presents a few specific examples of cored carbon electrodes ,and current densities which have been used in high intensity alternating current arcs to produce light of high intrinsic brilliancy of the character described. Each electrode contained approximately 23% by weight of flame-producing material.

' Current den- Electrode diameter, Arc Current Arc length sity, amperes millimeters voltage amperes in per square r inch The light sources from previously known alter-' nating current arcs proposed for the projection of motion pictures and the like are of low intrinsic brilliancy (approximately to less than candle power per square millimeter). By comparison, the novel light-producing system of this invention is characterized by light sources having an intrinsic brilliancy of between 235 and 290 particular installation, light of high intrinsic brilliancy was. produced in accordance withthis invention using cored carbon electrodes, each 8 millimeters in diameter and containing 23% by weight of flame-producing .material. Alternat- 1 ing current was supplied to the electrodes from a 115 volt source using a 3.6 k. v. a. step-down transformer having a ratio of transformation of 2.7. The reactance of this transformer was sufiiciently high to insure good are stability.- The electrodes were adjusted to form a gap of approximately 0.35 inch and to produce an arc of 80 amperes and 28 volts. With arTc current of 80 amperes for the8 millimeter electrodes, the current density was about 1030 amperes per square inch. The power required to operate this system was about 2340 watts and its power factor was .about 0.67.

When the same arc of 80 amperesand 28 volts was obtained by connecting the electrodes directly to the 115 volt source of alternating current using a resistor in series with the electrodes, the system required 290% more power than was consumed by the installation described above. the other hand, when the same are of 80 amperes and 28,volts was established between electrodes connected directly'to the 115 volt source of alternating current using a reactor in series with the electrodes, the amount of power consumed was not substantially increased over that of the first described system, but the power factor of the load dropped to about 0.26.

- It will thus be seen that an arrangement ineluding a transformer as described makes possible the economical operation of a light-producing system in which cored carbon electrodes containing substantial amounts of flame-producing materials are energized at high current densities to produce an alternating current lowvoltage high-intensity arc of novel form and character which includes a source of steady and uniform light of high intrinsic brilliancy concentrated within a relatively small volume near each of the tips of the electrodes.

' The light produced as described herein is of special value in the projection of motion pictures because the concentrated sources adjacent the electrode tips may be readily focused either singly or together by means of the usual optical necessary for the projection of motion pictures from black and white and from certain types of color films. In these electrodes, the flame-producing material usually is combined with other materials to form the core. Thus, potassium or sodium silicates may be included to assist in maintaining a steady burning arc and to eliminate sputtering, but these substance are notused in amounts large enough to affect the color of the light produced. Cores for electrodes de+ signed to produce blue-white light of high intrinsic brilliancy may, for example, be-composed of 70 parts of a rare earth metal fluoride as the flame-producing material, 5 parts of potassium silicate and 25 parts .of carbon.

If light which predominates in color other than blue-white is desired, any of the well-known color 'all of the rare earth metal compounds.

flame-producing alkali metal or alkaline earth metal compounds may be substituted for a part or For example,- in the projection of motion pictures from certain types of color films, a modified blue-white light source is required which contains light of the red portion of the spectrum. Light of high intrinsic brilliancy and of suitable color for this purpose may be produced in accordance with our invention by including, in the flame-producing material of the cored carbon electrodes, either alone or in combination with the rare earth metal compounds thereof, materials known to yield red coloration, such as compounds of strontium or calcium.'- Thus, it will be apparent that control of the color characteristics of the light produced in accordance with this invention may be accomplished by appropriate additions to the flameproducing materials of the electrode.

The materials used in modifying the composition of the electrode cores not only influence the color-ofthe arc I2, but they also modify similarly the color of the intense luminescent light sources 26 at the tips of the electrodes l0 and II, to produce the desired concentrated light of high intrinsic brilliancy which is a feature of the novel are form of this invention.

It will be evident-that the principles of the invention can be adapted to other systems where light of 'high intrinsic brilliancy is required in light-therapy, industrial irradiation and illumination and elsewhere. The invention is not limited in scope to the'particular systems described above but includes related light-producing systems embodying the principles of the invention and employing carbon electrodes having cores containing such metals as nickel, iron, cobalt,

lationship, each electrode having a core containing flame-producing material, the weight of such material. in a core being more than about per cent of the weight of said electrode; anda source ofalternating current in circuit with said electrodes and providingin each elect-rode a current density greater than about 550 amperes per square inch.

2. The combination of a circuit including similar .cooperating carbon electrodes having substantially the same diameter and composition arranged to have an arc established therebetween; each electrode'having a core containing flame-producing material comprising more than about 10 percent. of the entire weight of said electrade; and means, including a source of alternating current connected to and having a current density greater than about 550 amperes per square inch in said electrodes, for producing separate sources of light of high intrinsic brilliancy confined within a relatively small volume near each of the tips of said electrodes.

3. A method of producing a concentrated steady and uniform light source of relatively small volume near each of the tips of a pair of electrodes which comprises connecting to a source of alternating current, and in arc-producing-relation, two similar electrodes eachhaving a core vof substantlally the same flame-producing material and of substantially the same diameter, and operating said electrodes at an alternating current density within the range sufiicient to produce separate light sources of' high weight of said electrode, which comprises passing an alternating current through said electrodes capable of producing therein a current density of at least 550 amperes per square inch toprovide an arc therebetween, such are being characterized by the fact that a source of light of high intrinsic brilliancy is produced and confined within a.relativelysmal1 volume about each oi. the

electrode tips and with the ends of said electrodes remaining substantially flat.

5. A method of producing steady and uniform concentrated light sources or relatively small volume near each of the tips of a pair of electrodes which comprises placing in arc producing relation two similar carbon electrodes having cores oi substantially identical flame-producing material, and of substantially the same diameter, and passing throughsaid electrodes an alternating current of sufiicient intensity tov provide a current density of at least 550 amperes per squar inch in each electrode.

6. A method of producing steady and uniform concentrated light sources of relatively sma1l volume near each of the tipsof am. of electrodes which comprises placing in are producing relation two similar carbon electrodes of substantially thesame diameter, each elect e having a core of substantially the same flame producing material, the weight of such material in a core being more than about 10 per cent of the weight of said electrode; and passing through said electrodes an alternating current of sufilcient intensity to provide a current density or at. least 700 amperes per square inch in each electrode.

7. A method of producing a steady and uniform electric are which comprises positioning cored carbon electrodes in spaced cooperating relation, each electrode containing flame-producing material; and causing'sufllcient alternating current .to fiow between said electrodes to produce a current density of at least 550 amperes per square inch in each electrode, whereby separate brilliant portions of" luminescent material will be formed and confined within a relatively small volume near each of said electrode 'tips.

8. A method of producing a steady and uniform electric are which comprises positioning a pair of cored carbon electrodes in spaced cooperating relation, each electrode containing fiame-produc ing material including at least one rare earth metal compound; and causing sufiicient alternating current to flow between said electrodes to produce a current density oi! at least 550 amperes per square inch in each electrode to form s'epa-' rate brilliant portions of luminescent blue-white material confined within a relatively small volume near each of the electrode tips.

9. A method as defined by claim 8, in which the flame-producing material comprises at least one rare earth metal compound from the cerium group, and the current density is at least 700 amperes per square inch in each electrode.

10. A method of producing a steady and uniform electric are which comprises positioning a pair of cored carbon electrodes in spaced coop-' erating relation, each electrode containing flameproducing material including at least ,one compound selected irom thegroup consisting of the compounds orv the and alkaline earth'metals; and causi'nisumcient alternating currentto flow between said electrodes to produce a .current density of at least 550 amperes per square inch ineach electrode to form separate brilliant portions ofluminescent material confined within a.

relatively small volume near each of the electrode tips.

11. A method of producing a source of light of high intrinsic brilliancy which comprises placing the ends of similar carbon electrodes in arc-producing relation, each electrode having a' core contairiing flame-producing material, the weight'of such material in a core being from about 10% to about 35% o! the weight of said electrode; and passing an alternating current through said electrodes capable of producing therein a current density of about-100 to about 1200 amperes per square inch at a potential about 22 to about 31 volts to provide an are between said ends having a length in the neighborhood of about 0.19 to about 0.38 inch, said are being characterized by the fact that it provides a source of light or high small volume about each of the electrode tips.

12. A method as defined by' claim 11, in which the carbon electrodes are of the same diameter and the flame-producing material in the core of each electrode consists predominantly of a rare -earth metal compound.

,13. A method as defined by claim 11, in which the carbon electrodes are of the same diameter, the flame-producing material in-the core of each electrode comprlsesarare earth metal fluoride and consists of about 23% by weight of the electrocle, and the brilliant light source is blue-white in character.-

14. A method of producing a sourceof light of high intrinsic brilliancy which comprises placing the ends of two'cored carbon electrodes in arc-'- producing relation, at least one of the said elec-' trodescontaining a rare earth metal compound in'the core as a flame material, the weight of such material in "said core being more than about 10% of the weight 0; the said electrode; and passing alternating current through said electrodes capable of producing therein a current density of at least about 550 amperes per square inch to provide an are between said ends, said are being characterized by the fact that it provides a source or light or high intrinsic brilliancy confined with in a relatively small volume about the tip of the electrode containing the rare earthmetal compound and that the ends of said electrodes remain substantially fiat.-.

15. A method of producing a steady and uniform light source of high intrinsic brilliancy by means oi. a carbon electrode having a core containing flame-producing material amounting to more than 10% or the electrode weight which comprises placing said electrode in arc-producing relation with another cored carbon electrode of similar diameter and passing through said electrodes alternating current of sufiicient intensity to provide a current density in each electrode 01' at least about 550 amperes per square inch, the afc formed between said electrodes being characterized'by the fact that intrinsic brilliancy confined within a relatively 

